I'm not really sure how to word this so I'll detail the situation and hopefully I can not only get an answer but a name for this situation.
I was in a game with a regular group doing a one shot as some of our group wasn't going to be around today. We all had level three characters and I believe one was a Barbarian with the Path of the Totem Warrior which I believe they chose the bear.
So during a moment of combat the person fiddled with a bear amulet they had as they said they're entering a rage. The player described that as they did so a ethereal visage of a bear enshrouded their character.
The GM had immediately said Rage doesn't do that and the player became flustered. After the session, the kettle was still boiling. To the player, the GM was outing them and stopping them from being creative. To the GM, they were confused to what the player was doing and then tried to stop them. Apparently there had been a past with the player flavoring spells and abilities in ways unusual to how they are described as written or to how they can be applied.
Part of me thinks this falls into the "rule of cool" category but it also falls into confusion with mechanics, whether superficial or actually mechanical in nature. That and we had a new player with us, so I'm not sure if it was in attempt to stop potential misleading information.
Can a player choose to add such a description to their ability (and spells)?